In a variable speed, constant frequency (VSCF) power generating system, a brushless, synchronous generator is supplied variable speed motive power by a prime mover and develops variable frequency AC power at an output thereof. The variable frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant frequency AC power, which is then supplied over a load bus to one or more loads.
As is known, a generator of a VSCF system can be operated as a motor in a starting mode to convert electrical power supplied by an external AC power source into motive power which may in turn be provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator having a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it is necessary to provide power at a controlled voltage and frequency to the armature windings of the main generator portion and to provide field current to the main generator portion via the exciter portion so that the motive power may be developed.
In known systems, field current for the main generator portion is developed at a beginning of operation in a starting mode by providing AC power at a controlled frequency and voltage to a set of exciter field windings. AC power is developed in armature windings of the exciter by transformer action and such power is rectified by a rotating rectifier and provided to the field winding of the main generator portion. In order to prevent saturation of the exciter armature windings, it is necessary to provide the AC power to the exciter field windings at a certain volts-per-hertz ratio. In addition, in order to operate the generator as a motor at unity power factor, it is necessary to provide a certain voltage to the exciter field windings for a given armature current. In the case of a brushless generator which is designed to produce 400 hz., 115 volt power, it is necessary to provide 90 volts to the exciter field windings during operation in the starting mode. This voltage, in turn, must be provided at 1200 hz. to prevent saturation of the exciter armature windings. Since starting is usually accomplished using 400 hz., 115 volt AC power supplied by a ground cart or other external AC source, it has been found necessary to use an auxiliary inverter to convert such power into the required 1200 hz., 90 volt power for starting. In some applications, the use of an auxiliary inverter may be considered undesirable.
Shilling, et al., U.S. Pat. No. 4,743,777 discloses a starter generator system using a brushless, synchronous generator. The system is operable in a starting mode to produce motive power from electrical power provided by an external AC power source. An exciter of the generator includes separate DC and three-phase AC field windings disposed in a stator. When operating in a starting mode at the beginning of a starting sequence, the AC power developed by the external AC power source is directly applied to the three-phase AC exciter field windings. The AC power developed by the external AC source is further provided to a variable voltage, variable frequency power converter which in turn provides a controlled voltage and frequency to armature windings of a main generator. The AC power provided to the AC exciter field windings is transferred by transformer action to exciter armature windings disposed on a rotor of the generator. This AC power is rectified by a rotating rectifier and provided to a main field winding of the generator. The interaction of the magnetic fields developed by the main generator field winding and armature windings in turn causes the rotor of the generator to rotate and thereby develop the desired motive power.
When the generator is operated in a generating mode, switches are operated to disconnect the AC exciter field windings from the external AC source and to provide DC power to the DC exciter field winding.
Messenger, U.S. Pat. No. 3,908,161 discloses a brushless generator including three exciter field windings which are connected in a wye configuration and provided three-phase AC power during operation in a starting mode. The three-phase AC power induces AC power in an exciter armature winding which is rectified and applied to a main generator field winding. Main armature windings receive controlled AC power to in turn cause rotation of the generator rotor. Thereafter, the three exciter field windings are connected in series and provided DC excitation when operating in a generating mode.
Kilgore, et al. U.S. Pat. No. 3,809,914 discloses a starting system for a prime mover. An exciter of a slip ring generator driven by the prime mover is operated as a slip ring induction motor in response to the application of external AC power thereto. Specifically, the generator includes a three-phase exciter field winding which is provided AC power during starting. Also during starting, a control is connected through slip rings to a three-phase exciter armature winding which is disposed on a rotor of the generator. The current flowing in the exciter armature winding is controlled to cause the exciter to develop motive power which is transferred to the prime mover to bring it up to self-sustaining speed.